Assigning a custom post type to a custom page

You have misunderstand the purpose of add_submenu_page and it’s callback function.

If you take a look at WordPress Codex you will see that the purpose of this function , in your case the slides_page function, is to output the content of the page. ie output HTML code etc

$function
(callback) (optional) The function to be called to output the content for this page.
Default:

http://codex.wordpress.org/Function_Reference/add_submenu_page

You have to separate the presentation from the logic of your plugin.

You can register your custom posts when the user activates the plugin using the register_activation_hook hook.

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